1. Field of the Invention
This invention relates to polypropylene interior parts for autocars. More particularly it relates to polypropylene interior parts obtained by molding a propylene-ethylene block copolymer having an ethylene content of 3 to 12% by weight and a propylene homopolymer part occupying 70 to 95% by weight of the total polymerization quantity, the isotactic pentad fraction and MFR of this homopolymer part having therebetween a relationship of 1.00.gtoreq.P.gtoreq.0.015 log MFR +0.955.
2. Description of the Prior Art
As to the materials for interior parts for autocars, plastics have recently been more and more used particularly from the viewpoint of making autocars lightweight or saving their fuel cost. Further, as to the kind of plastics used, polyolefin resins, particularly polypropylene resins have been perferably used since these resins are superior in moldability, economy, appearance, strength, weathering resistance and durability of molded products.
As the materials for interior parts occupying a main position among parts for autocars, thermoplastic resins such as vinyl chloride resin, ABS resin, PS resin, modified PPO resin and polyacetal resin have so far been used, but since polypropylene resin is superior in moldability and other properties mentioned above it is notably advantageous in the reference to mass-productivity.
However, it has been found by research of the present inventors that interior parts for autocars (hereinafter abbreviated to interior parts) molded using so far generally known polypropylene resin (hereinafter abbreviated to polypropylene) have the following fatal problem: interior parts molded using polypropylene have a latent residual stress due to internal strain generated during the cooling process after molding; hence when the parts are fixed to the inside of the passenger compartment of autocars and then exposed to a high temperature of 70.degree. to 120.degree. C., the above internal strain is released to bring about a secondary deformation of the parts.
Interior parts are generally fixed at a number of points. In the case of plastic interior parts, due to a linear expansion behavior caused by the above high temperature exposure, a bulge amounting to 5 to 20 mm is formed between the fixed points. As to this bulge, due to a linear shrinkage behavior caused when the temperature inside the passenger compartment of autocars decreases from a high temperature down to room temperature or lower, it is possible to recover the original shape at the initial period when the number of times of the expansion shrinkage is small. However, when the expansion and shrinkage are repeated many times, a creep phenomenon occurs to make it impossible to recover the original shape and a "permanent bulge" occurs. When interior parts are fixed to the inside of the passenger compartment of autocars, the permanent bulge makes inferior the "fitting" of the interior parts to adjacent parts i.e. the physically adjoining state. In the case of polypropylene interior parts, too, it has been found that a serious problem of the above permanent bulge has been raised.
The present inventors have made extensive research in order to solve the above problems relative to the polypropylene interior parts, and as a result, have found that the above drawback of "permanent bulge" or "secondary deformation" can be overcome in the case of interior parts using a specified highly crystalline propylene-ethylene block copolymer in place of conventional polypropylene, particularly in the case of interior parts wherein the cross section perpendicular to the axial direction of the parts has a percentage opening of 20 to 48% (this percentage opening will be defined later)*.
As is apparent from the foregoing, an object of the present invention is to provide polypropylene interior parts for autocars having overcome the drawback of their permanent bulge or secondary deformation due to high temperature exposure. Another object of the present invention is to provide such interior parts having a suitable quantity of fillers blended therein.